Magnetoresistive random access memories (MRAMs) have been proposed as an alternative to conventional memory devices such as static random access memories (SRAM), dynamic random access memories (DRAM), and flash memories. MRAMs store data using a magneto resistance effect, which refers to a phenomenon wherein electrical resistance of a material changes with magnetic fields to which the material is subjected. As compared to these conventional memories, MRAMs are advantageous because of their fast speed, high integration density, low power consumption, radiation hardness, and endurance.
U.S. Pat. No. 6,545,906 to Savtchenko et al. discloses a conventional MRAM and the writing methods thereof. FIGS. 1-4, 7-8, 5-6 of U.S. Pat. No. 6,545,906 are reproduced herein as FIGS. 1-8, respectively.
FIG. 1 shows a memory cell 10 of an MRAM array 3. Memory cell 10 is sandwiched between a word line 20 and a digit line 30. Word line 20 and digit line 30 are perpendicular to each other and include conductive materials so that currents can pass therethrough.
Memory cell 10 includes a first magnetic region 15, a tunneling barrier 16, and a second magnetic region 17, wherein tunneling barrier 16 is sandwiched between first magnetic region 15 and second magnetic region 17. First magnetic region 15 has a synthetic anti-ferromagnetic (SAF) structure and includes a tri-layer structure 18, with an anti-ferromagnetic coupling spacer layer 65 sandwiched between two ferromagnetic layers 45 and 55. Anti-ferromagnetic coupling spacer layer 65 has a thickness 86 and ferromagnetic layers 45 and 55 have thicknesses 41 and 51, respectively. Second magnetic region 17 has a tri-layer structure 19, with an anti-ferromagnetic coupling spacer layer 66 sandwiched between two ferromagnetic layers 46 and 56. Anti-ferromagnetic coupling spacer layer 66 has a thickness 87 and ferromagnetic layers 46 and 56 have thicknesses 42 and 52, respectively. The thickness 86 of anti-ferromagnetic coupling spacer layer 65 is such that ferromagnetic layers 45 and 55 are anti-ferromagnetically coupled, i.e., a magnetic moment vector 57 of ferromagnetic layer 45 and a magnetic moment vector 53 of ferromagnetic layer 55 are anti-parallel to each other. Similarly, thickness 87 of anti-ferromagnetic coupling spacer layer 66 is such that ferromagnetic layers 46 and 56 are anti-ferromagnetically coupled, i.e., a magnetic moment vector 58 of ferromagnetic layer 46 and a magnetic moment vector 59 of ferromagnetic layer 56 are anti-parallel to each other. FIG. 1 also shows a resultant moment vector 40 of magnetic region 15, i.e., the combination of moment vector 57 of ferromagnetic layer 45 and moment vector 53 of ferromagnetic layer 55, and a resultant moment vector 50 of magnetic region 17, i.e., the combination of moment vector 58 of ferromagnetic layer 46 and moment vector 59 of ferromagnetic layer 56.
FIG. 2 shows magnetic moments in memory cell 10 with respect to the directions of word line 20 and digit line 30. In FIG. 2, word line 20 is shown to run horizontally along an x-axis and digit line 30 is shown to run vertically along a y-axis. Tri-layer structure 18 has two easy axes: a positive easy axis at an angle of 45° with both the positive x-axis direction and the positive y-axis direction and a negative easy axis is at an angle of 45° with both the negative x-axis direction and the negative y-axis direction. An easy axis is defined as an intrinsic orientation of magnetic dipole moments of an anisotropic material in the absence of an external magnetic or biasing field. Thus, moment vector 57 of ferromagnetic layer 45 is in the positive easy axis direction, and magnetic moment vector 53 of ferromagnetic layer 55 is in the negative easy axis direction. Resultant magnetic moment vector 40 of magnetic region 15 is thus either in the positive easy axis direction or the negative easy axis direction. FIG. 2 shows the resultant magnetic moment vector 40 of magnetic region 15 to be in the negative easy axis direction. Although not shown in FIG. 2, it is assumed that moment vector 58 of ferromagnetic layer 46 is in the negative easy axis direction, moment vector 59 of ferromagnetic layer 56 is in the positive easy axis direction, and resultant magnetic moment vector 50 of magnetic region 17 is in the negative easy axis direction.
Generally, magnetic region 15 is a free ferromagnetic region and magnetic region 17 is a pinned ferromagnetic region, i.e., magnetic moments in magnetic region 15 are free to rotate when an external magnetic field is applied, while magnetic moments in magnetic region 17 do not rotate when a moderate external magnetic field is applied.
An electron tunneling barrier of tunneling barrier 16 and, therefore, electrical resistance of memory cell 10, change with magnetic fields. For example, when moment vector 53 of ferromagnetic layer 55 and moment vector 58 of ferromagnetic layer 46 are parallel to each other, tunneling barrier 16 has a low electron tunneling barrier and memory cell 10 has a low resistance. When moment vector 53 of ferromagnetic layer 55 and moment vector 58 of ferromagnetic layer 46 are anti-parallel to each other, tunneling barrier 16 has a high electron tunneling barrier and memory cell 10 has a high resistance. Thus, by altering the magnetic moment vectors of magnetic region 15, a bit of datum may be stored in memory cell 10, with high and low electrical resistances of thereof respectively defining a bit of “1” or “0”, or the converse.
To read memory cell 10, a voltage may be applied across memory cell 10 and a current therethrough is sensed. Memory array 3 may include at least one dummy memory cell having the same structure as memory cell 10. The dummy memory cell may have magnetic moments configured in a certain manner and unaltered during operation of memory array 3. The same voltage applied across memory cell 10 may be applied to the dummy memory cell and a current through the dummy memory cell is sensed and used as a reference current. Then, the current through memory cell 10 is compared with the reference current and the difference indicates whether memory cell 10 has a “0” or “1” stored therein.
Currents provided in word line 20 and digit line 30 induce magnetic fields. For example, with reference to FIGS. 1 and 2, a word current 60 (IW) through word line 20 induces a circular word magnetic field 80 (HW), and a digit current 70 (ID) through digit line 30 induces a circular digit magnetic field 90 (HD). The strength of magnetic fields HW and HD are respectively proportional to word current IW and digit current ID. It is assumed that word line 20 is above memory cell 10 and digit line 30 is below memory cell 10. Thus, when word current IW is positive, HW is in the positive y-axis direction in the plane of memory cell 10; when digit current ID is positive, HD is in the positive x-axis direction in the plane of memory cell 10.
Under magnetic fields HW and HD, electron spins in ferromagnetic layers 45 and 55 flop (so-called “spin flop”), and moment vectors 57 and 53 may rotate. Consequently, resultant magnetic moment vector 40 also rotates. When resultant magnetic moment vector 40 rotates by 180°, moment vector 53 of ferromagnetic layer 55 and moment vector 58 of ferromagnetic layer 46 are anti-parallel to each other, and memory cell 10 is said to be switched, either from “0” to “1”, or from “1” to “0”, depending on how “0” and “1” are defined.
FIG. 3 shows the simulated switching behavior of tri-layer structure 18 under different magnetic fields HW and HD, where HW and HD are generated by a pulse of word current IW and a pulse of digit current ID provided in a sequence 100 shown in FIG. 4. Particularly, as shown in FIG. 4, at time t0, both IW and ID are 0; at time t1, IW is supplied; at time t2, ID is also supplied; at time t3, IW is turned off; and at time t4, ID is also turned off. In FIG. 3, the x-axis is the amplitude of word magnetic field HW in Oersteds, and the y-axis is the amplitude of digit magnetic field HD in Oersteds.
FIG. 3 shows three operation regions of memory cell 10. First, in a “no-switching” region 92, one or both of IW and ID are small and the corresponding one or both of HW and HD are weak. Memory cell 10 does not switch state.
A second operation region of memory cell 10 is referred to as a “direct” writing region, where both IW and ID are large and HW and HD are strong. IW and ID, when applied in sequence 100, directly write to memory cell 10. For example, if both IW and ID are positive, after IW and ID are provided in sequence 100, a bit of “1” is written into memory cell 10, regardless of whether the initial state of memory cell is “0” or “1”. Similarly, if both IW and ID are negative, after IW and ID are provided in sequence 100, a bit of “0” is written into memory cell 10. Under direct writing, an imbalance between moment vectors 53 and 57, i.e., resultant moment vector 40, is significant.
FIGS. 5(a)-5(e) and 6(a)-6(e) illustrate the examples of directly writing to memory cell 10.
FIGS. 5(a)-5(e) illustrate an example of directly writing “1” into memory cell 10, which has an initial state of “0”, by applying a positive word current IW and a positive digit current ID. It is assumed that moment vector 53 of ferromagnetic layer 55 is in the negative easy axis direction, moment vector 57 of ferromagnetic layer 45 is in the positive easy axis direction, and moment vector 53 is stronger than moment vector 57. It is also assumed that moment vector 58 of ferromagnetic layer 46 is in the negative easy axis direction, moment vector 59 of ferromagnetic layer 56 is in the positive easy axis direction, and moment vector 58 is stronger than moment vector 59. It is further assumed that memory cell 10 has a bit of “0” stored therein when moment vector 53 of ferromagnetic layer 55 and moment vector 58 of ferromagnetic layer 46 are parallel with each other, and has a bit of “1” stored therein when moment vector 53 of ferromagnetic layer 55 and moment vector 58 of ferromagnetic layer 46 are anti-parallel with each other.
As FIG. 5(a) shows, at time t0, moment vector 57 of ferromagnetic layer 45 is in the positive easy axis direction. Moment vector 53 of ferromagnetic layer 55 is in the negative easy axis direction. Because moment vector 53 is assumed to be stronger than moment vector 57, resultant magnetic moment vector 40 is also in the negative easy axis direction. Memory cell 10 has a bit of “0” stored therein.
Referring to FIG. 5(b), at time t1, positive word current IW is provided, generating word magnetic field HW in the positive y-axis direction. Because magnetic moments tend to align with external magnetic fields to lower the energy of a system, both moment vectors 53 and 57 tend to rotate towards the direction of HW, i.e., the positive y-axis direction. However, due to the anti-ferromagnetic coupling between ferromagnetic layers 45 and 55, and also due to the fact that moment vector 53 is stronger than moment vector 57, both moment vectors 53 and 57 rotate in the clockwise direction, with resultant magnetic moment vector 40 rotating towards the direction of the magnetic moment vector of the external magnetic field, i.e., the positive y-axis direction.
Referring to FIG. 5(c), at time t2, positive digit current ID is provided, generating digit magnetic field HD in the positive x-axis direction. Assuming HW and HD have the same magnitude, a magnetic field vector of the total external magnetic field is in the positive easy axis direction. For the same reasons stated above, both moment vectors 53 and 57 further rotate in the clockwise direction, and resultant magnetic moment vector 40 rotates towards the direction of the magnetic moment vector of the external magnetic field.
Referring to FIG. 5(d), at time t3, word current IW is turned off. The external magnetic field has only one component, i.e., HD, in the positive x-axis direction. Moment vectors 53 and 57 and resultant magnetic moment vector 40 further rotate in the clockwise direction. Moment vector 53 is now closer to the positive easy axis, and moment vector 57 is closer to the negative easy axis. Resultant magnetic moment vector 40 is close to the positive x-axis.
Finally, as FIG. 5(e) shows, at time t4, digit current ID is also turned off. The external magnetic field is zero. Moment vectors 53 and 57 align with the easy axes. Because moment vector 53 was closer to the positive easy axis, and moment vector 57 was closer to the negative easy axis prior to time t4, moment vector 53 aligns with the positive easy axis, and moment vector 57 aligns with the negative easy axis. In other words, both moment vectors 53 and 57 have rotated 180° from their initial states in FIG. 5(a). As a result, moment vector 53 is anti-parallel with moment vector 58 of ferromagnetic layer 46, and a bit of “1” is written in memory cell 10.
FIGS. 6(a)-6(e) illustrate an example of directly writing “1” into memory cell 10, which has an initial state of “1”. As FIG. 6(a) shows, at time to, moment vector 53 is in the positive easy axis direction. Moment vector 57 is in the negative easy axis direction. Resultant magnetic moment vector 40 is in the positive easy axis direction. Memory cell 10 has a bit of “1” stored therein.
As shown in FIG. 6(b), at time t1, positive word current IW is provided, generating word magnetic field HW in the positive y-axis direction. Because moment vector 53 is stronger, there will only be minimal clockwise rotation of moment vectors 53 and 57. But resultant magnetic moment vector 40 rotates counterclockwise towards HW.
As shown in FIG. 6(c), at time t2, positive digit current ID is provided, generating digit magnetic field HD in the positive x-axis direction. Moment vectors 53 and 57 rotate in the clockwise direction, and resultant magnetic moment vector 40 rotates in the direction of the magnetic field vector of the external magnetic field, which is in the positive easy axis direction.
As shown in FIG. 6(d), at time t3, word current IW is turned off. The external magnetic field has only one component, i.e., HD, in the positive x-axis direction. Resultant moment vector 40 further rotates clockwise towards HD. Because moment vector 53 was closer to the positive easy axis, and moment vector 57 was closer to the negative easy axis prior to time t4, moment vector 53 rotates counterclockwise towards the positive easy axis, and moment vector 57 rotates counterclockwise towards the negative easy axis.
Then, as shown in FIG. 6(e), when digit current ID is also turned off at time t4, moment vectors 53 and 57 return to their original states and align along the easy axes. As a result, a bit of “1” is written in memory cell 10.
Negative currents IW and ID may be provided to write a bit of “0” into memory cell 10. The behavior of memory cell 10 during direct writing of a bit of “0” is similar to those described above with reference to FIGS. 5(a)-5(e) and FIGS. 6(a)-6(e), except that the polarities of the magnetic moments are opposite, and is therefore not described herein.
When IW and ID are even larger and HW and HD are even stronger, memory cell 10 operates in a third region called “toggle” region 97, as shown in FIG. 3. When large positive currents IW and ID are provided in sequence 100, the state of memory cell 10 switches, i.e., an initial state of “0” switches to “1” and an initial state of “1” switches to “0”. This writing method is referred to as “toggle writing.” Under toggle writing, because strong HW and HD are provided, the imbalance between moment vectors 53 and 57, i.e., resultant moment vector 40, is insignificant or weak.
FIGS. 7(a)-7(e) illustrate an example of toggle writing to memory cell 10 with an initial state of “1”.
As FIG. 7(a) shows, at time to, moment vector 53 of ferromagnetic layer 55 is in the positive easy axis direction. Moment vector 57 of ferromagnetic layer 45 is in the negative easy axis direction. Weak resultant magnetic moment vector 40 is in the positive easy axis direction. Memory cell 10 has a bit of “1” stored therein.
As shown in FIG. 7(b), at time t1, positive word current IW is provided, generating strong word magnetic field HW in the positive y-axis direction. Because HW is very strong, both moment vectors 53 and 57 rotate clockwise, and resultant magnetic moment vector 40 substantially aligns with the direction of HW. Particularly, both moment vectors 53 and 57 now point above the x-axis.
As shown in FIG. 7(c), at time t2, positive digit current ID is provided, generating strong digit magnetic field HD in the positive x-axis direction. Moment vectors 53 and 57 further rotate in the clockwise direction, and resultant magnetic moment vector 40 substantially aligns with the direction of the magnetic field vector of the external magnetic field, which is in the positive easy axis direction. Moment vector 53 is now between the positive x-axis and the bisector of the angle between the positive x-axis and the negative y-axis. Moment vector 57 is now between the positive y-axis and the bisector of the angle between the negative x-axis and the positive y-axis.
As shown in FIG. 7(d), at time t3, word current IW is turned off. The external magnetic field has only one component, i.e., HD, in the positive x-axis direction. Resultant moment vector 40 substantially aligns with HD. Moment vectors 53 and 57 further rotate in the clockwise direction. Moment vector 53 is now closer to the negative easy axis. Moment vector 57 is now closer to the positive easy axis.
Then, as FIG. 7(e) shows, digit current ID is also turned off at time t4. Because, prior to time t4, moment vector 53 was closer to the negative easy axis, and moment vector 57 was closer to the positive easy axis, moment vector 53 aligns with the negative easy axis, and moment vector 57 aligns with the positive easy axis. As a result, a bit of “0” is written in memory cell 10.
When memory cell 10 has an initial state of “0”, toggle writing with large positive currents IW and ID writes a bit of “1” into memory cell 10. FIGS. 8(a)-8(e) show the changes with time of moment vectors 40, 53, and 57, when IW and ID are provided in sequence 100 as shown in FIG. 4. The behavior of memory cell 10 during toggle writing of a bit of “1” is similar to those described above with reference to FIGS. 7(a)-7(e), except that the polarities of the magnetic moments are opposite, and is therefore not described herein.
Because during toggle writing, the state of memory cell 10 always changes, the initial state of memory cell 10 must be read and compared to the state to be written prior to performing toggle writing. If the initial state is the same as the datum to be written, no toggle writing is necessary. If the initial state is different from the datum to be written, toggle writing is performed. Thus, as compared to direct writing, toggle writing requires additional logic circuitry. However, because toggle writing only writes a memory cell when the state of the memory cell needs to be changed, toggle writing consumes less power.
Because toggle writing requires strong external magnetic fields HW and HD, large writing currents are needed. To alleviate this problem, Engel et al. proposed in U.S. Pat. No. 6,633,498 to adjust the magnitude of magnetic moment vector 50 of magnetic region 17 to generate a fringe (or stray) magnetic field as a bias magnetic field HBIAS in tri-layer structure 18, such that only weak magnetic fields HW and HD are required to toggle write memory cell 10. FIGS. 4 and 5 of U.S. Pat. No. 6,663,498 are reproduced herein respectively as FIGS. 9 and 10. As FIGS. 9 and 10 show, if positive HW and HD are used to write memory cell 10, a bias magnetic field HBIAS in a direction between the positive x-axis direction and the positive y-axis direction lowers the required values of HW and HD. Similarly, if negative HW and HD are used to write memory cell 10, a bias magnetic field HBIAS in a direction between the negative x-axis direction and the negative y-axis direction lowers the required values of HW and HD. Consequently, lower currents IW and ID are required. The stronger the bias magnetic field HBIAS is, the lower the currents IW and ID may be.
However, a strong HBIAS may cause writing failure. Particularly, when HBIAS is strong, magnetization in end domains of ferromagnetic layers 45 and 55 is irregular, and memory cell 10 may fail to switch in response to writing currents IW and ID. FIGS. 11(a)-11(e) illustrate an example when the toggle writing method fails to write a bit of “1” into memory cell 10 having an initial state of “0” when HBIAS is strong.
FIG. 11(a) shows the state of memory cell 10 at time t0. A strong HBIAS is generated in the positive easy axis direction. Because of the strong HBIAS, magnetization in end domains of ferromagnetic layers 45 and 55 is so irregular that the magnetic moment vectors thereof, 57 and 53, may rotate counterclockwise and respectively approach or pass the y-axis, as shown in FIG. 11(a). Then, as FIG. 11(b) shows, at time t1, positive word current IW is provided, generating word magnetic field HW in the positive y-axis direction. Because moment vector 53 is close to the positive x-axis and moment vector 57 is close to negative x-axis, and a combination of HW and HBIAS is in a direction between the positive y-axis and positive x-axis, moment vectors 53 and 57 further rotate counterclockwise. As FIG. 11(c) shows, at time t2, positive digit current ID is provided, generating digit magnetic field HD in the positive x-axis direction. In response, moment vectors 53 and 57 start to rotate in the clockwise direction. As FIG. 11(d) shows, at time t3, when word current IW is turned off, moment vectors 53 and 57 further rotate in the clockwise direction. Now moment vector 53 is closer to the negative easy axis and moment vector 57 is closer to the positive easy axis. As FIG. 11(e) shows, at time t4, when digit current ID is also turned off, moment vectors 53 and 57 return to their original positions as in FIG. 11(a). Thus, moment vectors 53 and 57 rotate in the wrong direction under HW because of the strong bias field HBIAS, and memory cell 10 fails to switch after IW and ID are provided in sequence 100 of FIG. 4.
When Memory cell 10 is scaled down, and magnetic regions 15 and 17 are very small, the above-described problem worsens, because irregularities of the magnetic field in ferromagnetic regions 15 and 17 increase. As a result, it is difficult to reduce writing currents IW and ID to a satisfactory level.